Flip chip is a method for directly interconnecting semiconductor dies and a substrate with solder balls (or solder bumps). During assembly, solder balls are first deposited onto a semiconductor die. A solder mask is formed on the substrate (e.g., a printed circuit board) to define a plurality of connection sites. The semiconductor die with the solder balls is then flipped over to align the solder balls with corresponding connection sites on the substrate. The solder balls are then reflowed to complete the interconnection.
One drawback of the foregoing flip chip technique is that the solder balls tend to uncontrollably collapse during reflow when the substrate is a lead frame having a plurality of lead fingers. The collapsed solder balls may cause various structural, functional, and/or other types of damages to the resulting microelectronic package. For example, adjacent solder balls may come in contact with one another to short circuit the semiconductor die and/or the substrate.
One conventional solution is to form a solder mask on the lead frame as on a printed circuit board. However, forming a solder mask on the small lead fingers is difficult, time consuming, and costly. Accordingly, there is a need for improved flip chip techniques that can at least reduce or eliminate the risk of uncontrollable collapse of solder balls during reflow.